Problem: The scale on a map is 2cm : 9km. If two cities are 8cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 2cm on the map corresponds to an actual distance of 9km. A distance of 8cm on the map is the same as 4 $\cdot$ 2cm. The actual distance between the two cities is 4 $\cdot$ 9km, or 36km.